DE102021105023B4 - Discharging circuit for discharging a capacitance, method for discharging a capacitance via the discharging circuit and electrical device with such a discharging circuit - Google Patents

Abstract

The application describes a discharge circuit (100) for discharging a capacitance (1) with - an input (101) for the electrical connection of the capacitance (1), - a transistor (3) whose drain connection (3D) is connected via a discharge resistor (2nd ) having a first input terminal (101a) and its source terminal (3S) connected to the second input terminal (101b),- a gate charging resistor (9) connecting the first input terminal (101a) to a gate terminal (3G) of the Transistor (3) connects,- a thermistor (4) thermally coupled to the discharge resistor (2) and/or the transistor (3), and- a switch (6) which connects the gate connection (3G) to the second input connection ( 101b) connects, and- a trigger circuit (5) which is designed to control the switch (6) depending on a temperature of the thermistor (4). The discharge circuit (100) is designed to be supplied from a voltage present at the input terminals (101a, 101b), the switch (6) and/or a combination of the trigger circuit (5) and the switch (6) being set up to suddenly change the switching state of the switch (6) from the blocking to the conducting switching state in order to cause a transient change from a first operating state BZ1 of the discharge circuit (100) when the transistor (3) is turned on to a second operating state BZ2 of the discharge circuit (100). blocking transistor (3) to generate. The application also discloses a method for discharging a capacitance and an electrical device with such a discharge circuit.

Description

Technical field of the invention

The invention relates to a method for discharging a capacitance and a discharge circuit suitable for carrying out the method. The invention also relates to an electrical device with such a discharge circuit.

State of the art

In the case of electrical devices, it is often required for safety reasons that, after the device has been switched off, all components that can be touched from the outside have no electrical voltage or only have a non-dangerous electrical voltage. It usually follows from this that the capacities of the device must be discharged in a controlled manner, if necessary within a specified period of time, after it has been switched off. In concrete terms, for example, the input capacitances of a photovoltaic (PV) inverter must be discharged within a specified period of time if, for safety reasons, it is disconnected from the PV modules of a PV generator assigned to it on the DC side and from an energy supply network on the AC side. Because the input capacitances are discharged, it is also ensured that DC supply lines connected to the input capacitance between the PV inverter and the PV generator are voltage-free, at least when an isolating device close to the generator is open. In this case, the input capacitances must also be safely discharged when the electrical device is switched off and/or is disconnected from its electrical supply during normal operation.

From Scripture EP 2 248 238 B1 a device for discharging a vehicle electrical system of an electric vehicle or an electrical component is known. The device has a switchable resistor which is formed from a transistor and a PTC resistor which is connected to a source terminal of the transistor and thermally coupled to the transistor. In this case, the control connection of the transistor is connected to a mains voltage.

The pamphlet DE 10 2018 006 054 A1 discloses a device for discharging an electrical energy store via a discharging path comprising an actuating device and an adjusting device. When the electrical energy store is at least partially discharged, the temperature of the actuating device changes as a result of the discharge. The actuating device is connected to a control device in a signal-conducting manner and can be brought into an electrically non-conductive open position and into an electrically conductive closed position as a function of a control signal from the control device. The control signal is dependent on the temperature of the actuator.

Document WO 2019/158748 A1 discloses a discharge device for actively discharging an electrical network or an electrically operated unit, comprising a discharge circuit with a current-limiting resistor and a first switch for connecting a component to be discharged to a reference potential indirectly via the current-limiting resistor. The discharge circuit also includes a limiting circuit arranged on the control terminal side of the first switch for limiting heating that occurs at the first switch and/or the current-limiting resistor during discharge operation. The limiting circuit has an NTC resistor that is thermally coupled to the first switch and/or the current limiting resistor.

The disadvantage of these concepts is that a relatively high amount of energy is stored, particularly with a high electrical DC voltage in the range above 400 V, which DC voltage is common, for example, in PV generators and thus also in the input capacitances of PV inverters connected to them. The discharge of this amount of energy in the given time is associated with a correspondingly large electrical power during the discharge process, which in turn places a high thermal load on the transistor, in the case of the EP 2 248 238 B1 also means the PTC resistance. In concrete terms, as the temperature of the PTC resistor rises or the temperature of the NTC resistor rises, a discharge current would be reduced in a continuous manner through the transistor and the PTC resistor or the current-limiting resistor. The voltage acts on the PTC resistor (the EP 2248238 B1 ) or at the current-limiting resistor (WO 2019/158748 A1) counteracts a control voltage of the transistor, the transistor being operated at least predominantly in its linear mode. This results in a high thermal load on the transistor. In addition, PTC resistors are usually only designed for a small number of thermal cycles and must be replaced after they have expired. The cycle stability of the PTC resistor, i.e. the number of permitted thermal cycles before the next component replacement, decreases as the thermal load within a cycle increases. However, reducing the discharge power to protect the transistor would result in the specified time for discharging the capacitance being exceeded.

object of the invention

The invention is based on the object of specifying an improved discharge circuit for a charged capacitance, with which the disadvantages mentioned above are avoided or at least significantly reduced. In particular, temperature-sensitive components of the discharge circuit, for example thermistors and semiconductor switches, should experience the lowest possible thermal load during a discharge process. In doing so, it must be ensured that the charged capacity is discharged within the specified period of time down to a DC voltage that is harmless when touched - usually < 60 V. The discharge circuit should be designed to carry out the discharge of the capacitance even without an external energy supply and should also be able to be produced as cost-effectively as possible. It is also an object of the invention to provide a method for discharging a capacitance using the discharge circuit. In addition, the object of the invention is to provide an electrical device in which the device's capacitances are discharged in a way that protects the components, particularly in a state in which the device is disconnected from its power supply.

solution

The object of demonstrating an improved discharge circuit is achieved according to the invention with the features of independent patent claim 1 . The object of demonstrating a method for discharging a capacitance is achieved according to the invention with the features of independent claim 13 . The object of demonstrating an electrical device in which a device-internal capacitance is discharged in a way that protects the components even when the device is disconnected from a power supply is achieved according to the invention with the features of independent claim 16 . Advantageous embodiments of the discharge unit are specified in claims 2 to 12, advantageous embodiments of the method are specified in claims 14 and 15. An advantageous embodiment of the electrical device is described in claims 16 and 17.

Description of the invention

• - einen Eingang mit einem ersten Eingangsanschluss und einem zweiten Eingangsanschluss zum Verbinden mit jeweils einem Anschluss der Kapazität,
• - einen Transistor, dessen Drain-Anschluss über einen Entladewiderstand mit dem ersten Eingangsanschluss und dessen Source-Anschluss mit dem zweiten Eingangsanschluss verbunden ist,
• - einen Gate-Ladewiderstand der den ersten Eingangsanschluss mit einem Gate-Anschluss des Transistors verbindet,
• - einen thermisch mit dem Entladewiderstand und/oder dem Transistor gekoppelten Thermistor. Die Entladeschaltung umfasst weiterhin
• - einen Schalter mit einem leitenden und einem sperrenden Schaltzustand, der den Gate-Anschluss mit dem zweiten Eingangsanschluss verbindet, und ausgelegt ist, seinen Schaltzustand in Reaktion auf ein Triggersignal zu verändern, und
• - eine Triggerschaltung zur Steuerung des Schalters, die ausgelegt ist, das Triggersignal für den Schalter in Abhängigkeit einer Temperatur des Thermistors zu erzeugen, und somit den Schalter in Abhängigkeit der Temperatur des Thermistors zu steuern. Die Entladeschaltung ist dadurch gekennzeichnet, dass sie eingerichtet ist, aus einer an den Eingangsanschlüssen anliegenden Spannung versorgt zu werden, und dass
• - der Schalter und/oder eine Kombination aus der Triggerschaltung und dem Schalter eingerichtet ist, den Schaltzustand des Schalters in Reaktion auf das Triggersignal, und somit auch in Abhängigkeit der Temperatur des Thermistors, sprungartig von dem sperrenden in den leitenden Schaltzustand zu verändern, um einen transienten Wechsel von einem ersten Betriebszustand BZ1 der Entladeschaltung bei durchgesteuertem Transistor in einen zweiten Betriebszustand BZ2 der Entladeschaltung bei sperrendem Transistor zu generieren.

A discharge circuit according to the invention for discharging a capacitance comprises:

• - an input with a first input connection and a second input connection for connection to a respective connection of the capacity,
• - a transistor whose drain terminal is connected to the first input terminal via a discharge resistor and whose source terminal is connected to the second input terminal,
• - a gate charging resistor which connects the first input terminal to a gate terminal of the transistor,
• - a thermistor thermally coupled to the discharge resistor and/or the transistor. The discharge circuit further includes
• - a switch with a conducting and a blocking switching state, which connects the gate terminal to the second input terminal, and is adapted to change its switching state in response to a trigger signal, and
• - a trigger circuit for controlling the switch, which is designed to generate the trigger signal for the switch depending on a temperature of the thermistor, and thus to control the switch depending on the temperature of the thermistor. The discharge circuit is characterized in that it is set up to be supplied from a voltage present at the input terminals, and that
• - the switch and/or a combination of the trigger circuit and the switch is set up to suddenly change the switching state of the switch from the blocking to the conducting switching state in response to the trigger signal, and thus also as a function of the temperature of the thermistor, in order to to generate a transient change from a first operating state BZ1 of the discharge circuit when the transistor is turned on to a second operating state BZ2 of the discharge circuit when the transistor is off.

The transistor can be in the form of a field-controlled transistor. In particular, this can be a field effect transistor (FET). As an alternative to this, however, the field-controlled transistor can also be in the form of an insulated gate bipolar transistor (IGBT). The description is given throughout with terminal designations related to an FET. In the case of an IGBT, the following assignments apply to the connection designations of the transistor: the source connection of the FET corresponds to an emitter connection of the IGBT, the drain connection of the FET corresponds to a collector connection of the IGBT. The gate terminal usually retains its designation.

• - Betreiben der Entladeschaltung in Abhängigkeit einer Temperatur des Thermistors in einem aus dem ersten BZ1 und dem zweiten Betriebszustand BZ2,
• - wobei die Entladeschaltung stationär in dem ersten Betriebszustand BZ1 betrieben wird, wenn eine Temperatur des Thermistors unterhalb eines Temperatur-Schwellwertes T_TH liegt, und
• - wobei die Entladeschaltung mit einem sich wiederholendem Wechsel zwischen dem ersten Betriebszustand BZ1 und dem zweiten Betriebszustand BZ2 betrieben wird, wenn die Temperatur des Thermistors größer oder gleich dem Temperatur-Schwellwert T_TH ist, so dass der Transistor in wiederholter Art und Weise zwischen dem durchgesteuerten und dem gesperrten Zustand wechselt

In a method according to the invention for discharging a capacitance, the capacitance is discharged via the discharge circuit according to the invention. The discharge circuit has a first Operating state BZ1 with the switch open and the transistor turned on, and a second operating state BZ2 with the switch closed and the transistor blocked. The procedure includes the following steps:

• - Operating the discharge circuit depending on a temperature of the thermistor in one of the first BZ1 and the second operating state BZ2,
• - Wherein the discharge circuit is operated in a stationary manner in the first operating state BZ1 when a temperature of the thermistor is below a temperature threshold value T _TH , and
• - wherein the discharge circuit is operated with a repetitive change between the first operating state BZ1 and the second operating state BZ2 when the temperature of the thermistor is greater than or equal to the temperature threshold value T _TH , so that the transistor in a repeated manner between the on and the locked state changes

The discharge circuit according to the invention is designed and set up to be operated in accordance with the method according to the invention when a capacitance is connected to its input.

The invention makes use of the effect that linear operation of the transistor is prevented when the capacitance is discharged, but is at least reduced to the greatest possible extent. Rather, the transistor—and thus the discharge circuit—operates primarily in two operating states, the first operating state when the transistor is on and the second operating state when the transistor is off. A change from the first to the second operating state is initiated by the switch, in that the switch itself or the switch in conjunction with the trigger circuit suddenly changes its switching state from the blocking to the conducting state. Since the gate terminal of the transistor is connected to the second input terminal when the switch is in the on state, a transient change occurs in response thereto from the first operating state BZ1 of the discharge circuit when the transistor is turned on to the second operating state BZ2 of the discharge circuit when the transistor is off.

Both in the first operating state BZ1 and in the second operating state BZ2 of the discharge circuit, a power loss generated across the transistor is significantly reduced in comparison to a linear operation of the transistor, as is known from the cited prior art. Specifically, the discharge resistor is dimensioned such that in the first operating state BZ1, a large part of the power loss when the capacitance is discharged is converted in the discharge resistor and not in the transistor. In the second operating state BZ2, a discharge of the capacitance is temporarily not only reduced by the blocking transistor, but suppressed. Even when the transistor is off--and quasi-temporarily suppressed discharge of the capacitance--a power loss converted at the transistor is significantly reduced in comparison to a more permanent operation of the transistor in its linear range. The reason for this is that in the present case the power loss of the transistor is reduced to a considerable extent to its switching losses.

In the method according to the invention, the discharge circuit according to the invention is then operated in the first operating state BZ1 with the transistor switched on when a temperature of the thermistor thermally coupled to the transistor and/or the discharge resistor falls below the temperature threshold value T _TH . This results in a current flow in a discharge path, which is formed via a series connection of the discharge resistor and the transistor, the magnitude of which is predominantly limited by the discharge resistor. The discharge resistor, and to a lesser extent also the transistor, and therefore also the thermally coupled thermistor heat up in the process. If a temperature of the thermistor now reaches or exceeds the temperature threshold value T _TH , the discharge of the capacitance is not only reduced but at least temporarily suppressed by the discharge circuit being switched to the second operating state BZ2 with the transistor off by means of a sudden closing of the switch.

As long as the temperature of the thermistor corresponds to the temperature threshold value T _TH or is above the temperature threshold value T _TH and the capacitance is still charged, the discharge circuit is repeatedly charged at least briefly via the gate charging resistor with the switch in the first operating state BZ1, in order to then, if the temperature of the thermistor still reaches or exceeds the temperature threshold value T _TH in the first operating state BZ1, to switch back to the second operating state BZ2 with the transistor off by abruptly closing the switch. As well as in connection with 1 explained again in more detail, a transient change that repeats itself over time, in particular several times, occurs between the first operating state BZ1 with the transistor turned on and the second operating state BZ2 with the transistor turned off. In this case, the duration of the second operating state BZ2 is usually significantly longer than the duration of the first Operating state, which is why at temperatures of the thermistor above the temperature threshold value over time there is only a comparatively small discharge of the capacity. Only when the temperature of the thermistor has reduced again to such an extent that it is below the temperature threshold value T _TH is the first operating state BZ1 maintained for a longer period of time, at least maintained until the thermistor also permanently ( he) flowing discharge current has heated up again to a temperature which corresponds to the temperature threshold value T _TH or exceeds it.

In summary, both the method according to the invention and the discharging device according to the invention result in a permanent, uninterrupted discharge of the capacitance at temperatures of the thermistor below the temperature threshold value T _TH . At a temperature of the thermistor at or above the temperature threshold value T _TH , on the other hand, the discharge of the capacitance is predominantly suppressed over time, but is repeatedly interrupted by comparatively short discharge pulses. Most of the power loss in the discharge path is converted to the discharge resistor, but not to the transistor. Since the discharge resistor is arranged between the drain connection of the transistor and the first input connection of the discharge circuit, the transistor is not operated in its linear operating range, at least only for as short a time as possible. The discharge circuit can therefore also be used when discharging high DC voltages present between the input terminals. Specifically, the discharge circuit can be used, for example, with DC voltages above 400 V, advantageously above 1000 V, particularly preferably even above 1400 V. The discharge circuit and the method for its operation is suitable for ensuring sufficiently rapid discharge of the capacitance even with such high DC voltages, without subjecting the transistor to a high load, as would otherwise occur in the case of predominantly linear operation of the transistor. Such DC voltages are, for example, quite common at a DC input of a PV inverter, which is why the discharge circuits can also be used particularly advantageously in a PV inverter. The discharge circuit is electrically powered from the charged capacitance. It is therefore designed to operate properly in an electrical device even when the electrical device is disconnected from its normal electrical supply. Even if the capacitance cannot be discharged, for example because it is recharged from a DC source that is incorrectly still connected to the capacitance, the discharge circuit or its components are protected from damage or destruction as a result of the fault.

Advantageous refinements of the invention are specified in the following description and the dependent claims, the features of which can be used individually and in any combination with one another.

The method according to the invention can operate with only one temperature threshold value T _TH of the thermistor. In this case, there is an entry “into” the steady-state operation of the discharge circuit in the first operating state BZ1, as well as an exit “from” the steady-state operation of the discharge circuit in the first operating state BZ1 at the same temperature threshold value T _TH . In an advantageous embodiment of the method, however, the "exit from" the stationary operation of the discharge circuit in the first operating state BZ1 at the temperature threshold T _TH and the "entry into" the stationary operation of the discharge circuit in the first operating state BZ1 at a second temperature Threshold T _TH,2 take place. Stable and robust overall operation of the discharge circuit can be supported via the second temperature threshold value T _TH,2 and a hysteresis behavior associated therewith. In concrete terms, it can therefore be advantageous for the discharge circuit to be operated in the first operating state BZ1 only when the temperature of the thermistor has fallen below the second temperature threshold value T _TH,2 , which is lower than the temperature threshold value T _TH . In this case, operation of the discharge circuit at a temperature of the thermistor between the temperature threshold value T _TH and the second temperature threshold value T _TH,2 can depend on a temperature of the thermistor prevailing shortly beforehand. For example, operation may depend on whether the thermistor is actually cooling or heating.

In an advantageous variant of the method, the operation of the discharge circuit can last as a function of the temperature of the thermistor until a DC voltage falling across the capacitance and therefore present at the input terminals of the discharge circuit falls below a voltage threshold value UTH. The voltage threshold may correspond to a DC voltage close to 0V. However, it can also be sufficient if the capacitance has only discharged to a DC voltage that does not pose a risk if touched. In this case, the voltage threshold value UTH can be e.g. 12 V, 30 V, possibly also 60 V, depending on the relevant standards.

The switch of the discharge circuit can be designed as a bistable switch and can include a thyristor, for example. A bistable switch within the meaning of the application is in particular a switch that is free of a linear working range. It therefore includes a conducting and a blocking switching state, but no transition region between the conducting and the blocking switching state in which an ohmic resistance in a power path of the switch changes continuously or constantly. As an alternative to this, however, the switch can also have a linear operating range and, for example, comprise a transistor, in particular a MOSFET transistor or an IGBT transistor. In these cases, the trigger circuit can then be designed as a flip-flop, which ensures that the switch occurs in a sudden manner when its switching state changes, in particular when changing from the blocking to the conducting switching state, i.e. that the linear transition range that may be present quickly is passed through. In particular, the switch can change from its blocking to its conducting switching state within 10 μs, advantageously within 5 μs, particularly preferably within 1 μs.

In one embodiment, the discharge circuit can additionally have a voltage stabilization unit, which contains, for example, a zener diode or a parallel circuit made up of a zener diode and a buffer capacitor. In this case, the cathode of the zener diode is connected to the gate connection of the transistor and the anode of the zener diode is connected to the second input connection. In such an interconnection, the voltage stabilization unit is designed to charge its buffer capacitor by a current emanating from the first input connection via the gate charging resistor up to a value of a DC voltage predetermined by the zener diode. The buffer capacitor and the gate charging resistor are dimensioned in such a way that the buffer capacitor can be charged to the value specified by the zener diode within a minimum of 0.2 s and a maximum of 4 s, particularly preferably a minimum of 0.5 s and a maximum of 2 s is completed. In particular when the switch of the discharge circuit is formed by a thyristor, the gate charging resistor can advantageously be dimensioned in such a way that a current which is limited by it and emanating from the first input connection is less than a holding current of the thyristor is. This ensures that the switch designed as a thyristor, when it is in its conducting switching state, can be switched back to the blocking switching state due to the current flowing through it, which is limited by the gate charging resistor.

The thermistor may be formed as a component of the trigger circuit, in other words the trigger circuit may include the thermistor. The thermistor can include a PTC thermistor (also called a PTC resistor). As an alternative to this, however, it can advantageously also be in the form of a thermistor (also called an NTC resistor). A thermistor usually has a higher cycle stability than a PTC thermistor, so it can go through several load cycles within each of the load cycles under the same thermal load before it should be replaced for safety reasons. The thermistor, possibly also the further components or all components of the trigger circuit, can be arranged in the discharge path, which is formed via the series connection of discharge resistor and transistor. Since the thermistor is arranged in the discharge path, the discharge current flows through it at least in part. It therefore heats up not only because of its thermal coupling to the discharge resistor and/or the transistor, but also as a result of the discharge current itself. This can be advantageous with regard to a rapid response from the trigger circuit.

The discharge resistor arranged in the discharge path can be designed as a single discharge resistor. Alternatively, however, it is also possible for the discharge resistor to include a multiplicity of resistors connected in series and/or in parallel with one another. In the latter case, the power to be converted is distributed over several resistors. In this case, the thermistor can be thermally coupled to only one resistor--preferably a resistor arranged centrally within the plurality of resistors--or to a plurality of resistors.

The trigger circuit can be implemented in different ways. As an example, four advantageous options for implementing the trigger circuit are detailed in the 3a - 3c and 4 shown and explained, which is why reference is made here to the corresponding figures and descriptions of figures.

An electrical device according to the invention comprises a capacitance and a discharge circuit according to the invention acting on the capacitance. In this case, the electrical device is designed and set up, in particular its discharge circuit, for carrying out the method according to the invention. In particular, the electrical device is set up by means of the discharge circuit to discharge the device-internal capacitance in a state at which the device is disconnected from its power supply.

The electrical device can in particular be a photovoltaic (PV) inverter. The PV inverter can be designed as a PV inverter with only one DC input for connecting a PV string or as a so-called multi-string PV inverter with multiple DC inputs each having an input capacitance for connecting multiple PV strings. Depending on the type of PV inverter, the capacitance can therefore include one or more input capacitances of the PV inverter that are decoupled by diodes.

During normal operation of the device, it is often undesirable to discharge the capacitance of the electrical device, since this can impair the functionality of the device. The electrical device can therefore advantageously have a switching unit that acts on the gate connection of the transistor and can be controlled by a controller of the device, which is set up to suppress or enable a discharge of the capacitance depending on a control signal from the device-internal controller. The switching unit can contain, for example, a further switch arranged between the gate connection and the source connection of the transistor, which, in response to a control signal from the device-internal controller, connects the gate connection of the transistor to the source connection of the transistor with low resistance and thus Transistor of the discharge device permanently blocks, whereby a discharge of the capacity during operation of the device is deliberately suppressed. Alternatively, it is of course also possible to open the further switch in response to a corresponding control signal from the device-internal controller, as a result of which discharging of the device-internal capacitance can be deliberately initiated.

character list

• 1 eine Ausführungsform einer erfindungsgemäßen Entladeschaltung in einer Ausführungsform;
• 2 eine Ausführungsform eines erfindungsgemäßen Verfahrens in Form eines Flussdiagramms;
• 3a eine Triggerschaltung zur Ansteuerung zur Ansteuerung eines als Thyristor ausgebildeten Schalters der Entladeschaltung in einer ersten Ausführungsform.
• 3b eine Triggerschaltung zur Ansteuerung zur Ansteuerung eines als Thyristor ausgebildeten Schalters der Entladeschaltung in einer zweiten Ausführungsform.
• 3c eine Triggerschaltung zur Ansteuerung eines als Thyristor ausgebildeten Schalters der Entladeschaltung in einer dritten Ausführungsform.
• 4 eine Triggerschaltung in einer vierten Ausführungsform zur Ansteuerung eines als Transistor ausgebildeten Schalters der Entladeschaltung.

The invention is illustrated below with the aid of figures. From these show

• 1 an embodiment of a discharge circuit according to the invention in an embodiment;
• 2 an embodiment of a method according to the invention in the form of a flow chart;
• 3a a trigger circuit for driving for driving a thyristor designed as a switch of the discharge circuit in a first embodiment.
• 3b a trigger circuit for driving a switch designed as a thyristor of the discharge circuit in a second embodiment.
• 3c a trigger circuit for driving a switch, designed as a thyristor, of the discharge circuit in a third embodiment.
• 4 a trigger circuit in a fourth specific embodiment for driving a switch, designed as a transistor, of the discharge circuit.

character description

In 1 an embodiment of a discharge circuit 100 according to the invention is shown. The discharge circuit 100 has an input 101 with a first input connection 101a and a second input connection 101b, to which a capacitance 1 to be discharged is connected. A discharge path 10 is connected in parallel with the input 101 and the capacitance 1 and is formed as a transistor 3 from a series connection of a discharge resistor 2 and a field-controlled transistor (FET, here: exemplarily as a MOSFET). The FET 3 has its drain terminal 3D connected via the discharge resistor 2 to the first input terminal 101a. A source terminal 3S of the FET 3 is connected to the second input terminal 101b, which in turn is connected to a reference potential GND. Furthermore, the discharge circuit 100 has a series circuit made up of a gate charging resistor 9 and a voltage stabilization unit 7, which is formed by a parallel circuit made up of a zener diode ZD1 and a buffer capacitor C1. In this case, the gate charging resistor 9 is connected to the first input connection 101a and the anode of the zener diode ZD1 is connected to the second input connection 101b. The cathode of the zener diode ZD1—and thus also a connection point between the gate charging resistor 9 and the voltage stabilization unit 7—is connected to the gate connection 3G of the FET 3 .

A switch 6 is arranged in parallel with the zener diode ZD1. The switch 6 is controlled via a trigger circuit 5, which is symbolized by an arrow illustrated in dashed lines pointing towards the switch 6. The switch 6 is activated by the trigger circuit 5 as a function of the temperature of a thermistor 4, which is 1 is symbolized by an arrow directed towards the trigger circuit 5. In addition, the trigger circuit 5 can be set up so that the switch 6 is also driven as a function of a DC voltage U _GS between the gate connection 3G and the source connection 3S of the transistor 3 to perform. The thermistor 4, which is in 1 shown by way of example as a thermistor (NTC) is thermally coupled to the discharge resistor 2 and/or to the FET 3 . The thermal coupling of the thermistor 4 is in 1 symbolized by double arrows. Although the thermistor 4 and the trigger circuit 5 in 1 are each shown as separate components, the thermistor 4 may be an integral part of the trigger circuit 5.

In one embodiment, it is possible for the discharge circuit 100 - but not absolutely necessary - to arrange the thermistor 4 and at least parts of the trigger circuit 5, possibly also the entire trigger circuit 5, in the discharge path 10. Two possible variants of such an interconnection within the discharge path 10 are in 1 symbolized by the optional circuit positions 13, 14 shown in dashed lines with their respective connection points 13a, 13b and 14a. Circuit positions 13, 14 and their connection points are referred to in connection with 3a - 3c and 4 illustrated different embodiments of the trigger circuits 5 reference.

In 2 an embodiment of the method according to the invention is shown in the form of a flow chart, as with a discharge circuit installed in a device, for example a PV inverter, according to FIG 1 can be carried out. The method starts with a method step S1. The electrical device comprising the discharge circuit is initially operated in its normal mode in a method step S2. During normal operation, the discharge circuit 100 is disabled. In a third method step S3, the device is switched off, the discharge circuit 100 being activated automatically. This can be done, for example, by the absence of a control signal that is otherwise present during normal operation of the device at a switching unit that connects the gate connection 3G of the transistor 3 to its source connection 3S with low impedance, causing the device-internal capacitance 1 to discharge during normal operation of the device is suppressed. In a fourth method step S4, it is checked whether an input voltage U in present at the input 101 of the discharge circuit 100 falls below _a voltage threshold value UTH. If this is not the case, then the capacitance 1 connected to the input 101 is not yet sufficiently discharged and the method branches to a fifth method step S5. In the fifth method step S5 it is checked whether a temperature T of the thermistor 4 is lower than a temperature threshold value T _TH . If this is the case, the method branches to a sixth method step S6, in which the discharge circuit 100 is operated in the first operating state BZ1 with the transistor 3 switched on. Capacitance 1 is discharged by an uninterrupted flow of current. The method branches from the sixth method step S6 back to the fourth method step S4. If, on the other hand, the temperature T of the thermistor 4 is greater than or equal to the temperature threshold value T _TH during the check in the fifth method step S5, the method branches to a seventh method step S7, in which the discharge circuit 100 alternates between the second operating state BZ2 with the transistor 3 and changes back and forth in the first operating state BZ1 when the transistor 3 is switched on. In this case, the first operating state BZ1 is usually assumed to be significantly shorter than the second operating state BZ2. As a result, the capacity 1 is discharged by a cyclically interrupted discontinuous flow of current. However, since the time periods in which the first operating state BZ1 is assumed are usually significantly shorter than the time periods in which the second operating state BZ2 is present, there is no significant discharge of the capacitance 1 in the seventh method step S7. From the seventh method step S7, the method branches back to the fourth method step S4. If it is now determined in the fourth method step S4 that the DC voltage U present at the input 101 _is less than the voltage threshold value U _TH , then the capacitance 1 connected to the discharge circuit 100 is sufficiently discharged and the method starts in an eighth step Method step S8 ended.

A loop of method steps S4-S7 is shown in separate blocks in the flow chart in order to explain the function of the method. According to the method, however, it is possible, and in the case of a discharge circuit that operates largely analogously, it is also common for the method steps S4-S7 to be run through almost simultaneously within the loop.

In 3a a first embodiment of a trigger circuit 5 is shown, such as is used, for example, within the discharge circuit 100 in 1 can be used. The trigger circuit 5 has a parallel circuit made up of the PTC thermistor 21 and a series circuit made up of a second Zener diode ZD2 and a second resistor R2. A connection point 28 between the second zener diode ZD2 and the second resistor R2 is connected to a control connection 11G of the switch 6 designed as a thyristor 11 . The trigger circuit 5 can be arranged at the second circuit position 14 within the discharge path 10 of the discharge circuit 100 between the source connection 3S of the transistor 3 and the reference potential GND or the reference potential GND. This is about the in 3a also drawn connection points 14a, 14b symbol ized that the connection points within the 1 correspond to. In addition, an arrow is directed from the connection points 14a, 14b to the source connection 3G or the reference potential GND, which again characterizes the arrangement of the trigger circuit 5 within the discharge circuit 100.

The current flowing in the discharge path 10 flows through the PTC thermistor 21 as a thermistor 4 during the first operating state BZ1. It therefore heats up both as a result of the thermal coupling with the discharge resistor 2 and/or the transistor 3 and as a result of the current passing through it. The heating of the PTC thermistor 21 increases its ohmic resistance and thus the voltage drop between the connection points 14a, 14b. If the voltage drop exceeds a threshold value defined by the second zener diode ZD2, then the thyristor 11 receives a voltage signal at its control connection 11G and is fired, as a result of which it suddenly becomes conductive. Correspondingly, the control terminal 3G of the transistor 3 is connected to its source terminal 3S, as a result of which the buffer capacitor C1 of the voltage stabilization unit 7 is discharged and the transistor 3 is switched to its blocking state. After the buffer capacitor C1 has been discharged, the current limited by the gate charging resistor 9 flows from the first input terminal 101a through the thyristor 11. However, this is less than a holding current of the thyristor, as a result of which the thyristor 11 is switched back to its blocking state. The current limited by the gate charging resistor 9 now begins to charge the buffer capacitor C1 to the value set by the zener diode ZD1. The voltage at the gate terminal 3G of the transistor 3 increases accordingly and the transistor 3 is turned on again. If the temperature of the thermistor 4 continues to be at least equal to or exceeds the temperature threshold value TTH, the thyristor 11 is fired again immediately and switched to its conductive switching state. This alternating change is now repeated until the temperature of the thermistor 4 falls below the temperature threshold value T _TH , as a result of which the transistor 3 again remains permanently switched on.

If the capacity 1 is sufficiently discharged, the discharge circuit 100 is no longer supplied and the method ends automatically.

In 3b a second embodiment of the trigger circuit 5 is shown, which can also be arranged in the second circuit position 14 within the discharge path. It is similar in structure and functional behavior to that in 3a illustrated first embodiment. The differences from the first embodiment are therefore mainly presented below.

The second embodiment of the trigger circuit 5 has a parallel circuit made up of a third zener diode ZD3 and a series circuit made up of a thermistor 22 and a third resistor R3. A connection point 29 is connected between the thermistor 22 and the third resistor R3 and to a control connection 11G of the switch 6 designed as a thyristor 11 .

In the second embodiment, the voltage drop between the connection points 14a, 14b is specified by the third zener diode ZD3. When the thermistor 22 heats up, its ohmic resistance decreases and a voltage present at the connection point 29 increases relative to the reference potential GND. When a voltage threshold value required for the thyristor 11 is reached, the thyristor 11 is fired and switched to its conductive state. The further functional behavior, including the alternating change between the conducting and the blocking switching state of the thyristor 11, corresponds to that already in 3a described behavior, but with the difference that an increase in the voltage at the connection point 29 by a reduction in the ohmic resistance of the thermistor 22, and not as in 3a caused by an increase in the ohmic resistance of the PTC thermistor. Therefore, the basic functional behavior on the explanations in the description of 3a referred.

In 3c a third embodiment of the trigger circuit 5 is illustrated. The third embodiment of the trigger circuit 5 includes an optocoupler 23. The trigger circuit 5 is not completely arranged in the discharge path 10, but only partially. Specifically, a first in 3c part shown on the left at the first circuit position 13 between the transistor 3 and the discharge resistor 2 arranged, which in turn is symbolized by appropriately drawn connection points 13a 13b, which also the connection points 13a, 13b within the 1 correspond to. The first part of the trigger circuit 5 has a structure similar to that in 3B shown second embodiment of the trigger circuit and includes a fourth zener diode ZD4, which is connected in parallel to a series circuit of a fourth resistor R4 and a thermistor 22 as a thermistor 4. A series circuit comprising a fifth Zener diode ZD5 and a diode of the optocoupler 23 is arranged in parallel with the fourth resistor R4. A further part of the trigger circuit 5 includes a series circuit made up of a fifth resistor R5, a transistor associated with the optocoupler 23 and a sixth resistor R6. The series circuit is connected at one end (here first connection point 15a) to the first input connection 101a and at the other end (here: further connection point 15b) to the reference potential GND.

The part of the trigger circuit 5 shown on the left operates similarly to the second embodiment according to FIG 3b . When the thermistor 22 heats up, a voltage present at the connection point 30 increases relative to a voltage present at the connection point 13b. If the voltage at the connection point 30 exceeds a threshold value specified by the fifth zener diode ZD5, this results in a current flow through the diode of the optocoupler 23, as a result of which the—previously blocked—transistor of the optocoupler 23 is turned on. As a result, a DC voltage present at the further connection point 31 is increased relative to the reference potential present at the further connection point 15b, as a result of which the switch 6 designed as a thyristor 11 is fired and switched to its conductive switching state. Here, too, the functional behavior associated with the closing and opening of the switch 6 with regard to a shifting of the discharge circuit 100 into the first operating state BZ1 when the transistor 3 is turned on and an operation with an alternating change between the second and the first operating state BZ2, BZ1 is fundamental equal to the functional behavior of the first embodiment, which is why the explanations below 3a is referenced.

In 4 1 shows a fourth embodiment of the trigger circuit 5 which, in contrast to the previously described embodiments, is not arranged in the discharge path 10 of the discharge circuit 100. In contrast, it is connected to a first connection point 16a with the gate connection 3G of the transistor 3, which in 4 is symbolized by an arrow emanating from the connection point 16a and directed towards 3G. A second connection point 16b of the trigger circuit 5 is connected to the reference potential GND of the discharge circuit, which is 4 is symbolized by an arrow emanating from the second connection point 16b and directed towards GND. The fourth embodiment of the trigger circuit 5 is designed as a flip-flop and acts to control a transistor 12 as a switch 6. The trigger circuit 5 contains a so-called Schmitt trigger circuit 32. The Schmitt trigger circuit 32 has a resistance bridge made of a thermistor 22 as a thermistor 4, a seventh R7, an eighth R8 and a ninth resistor R9. A connection point of a series connection made up of the seventh resistor R7 and the thermistor 22 is connected to a positive input of an operational amplifier 35, and a connection point of a series connection made up of the eighth resistor R8 and the ninth resistor R9 is connected to a negative input of the operational amplifier 35. An output of the operational amplifier 35 is connected to the connection point of the seventh resistor R7 and the thermistor 22 via a tenth resistor R10 in the form of positive feedback. The Schmitt trigger circuit 32 is supplied from a voltage stabilization circuit 33 comprising a parallel connection of a sixth zener diode ZD6 and a second buffer capacitor C2. The voltage stabilizing circuit 32 is fed from a voltage present at the gate connection 3G of the transistor 3, specifically via a series circuit made up of an eleventh resistor R11 and a second diode D2, which is connected to the first connection point 16a of the trigger circuit 5. In addition, the output of the operational amplifier 35 is connected to a gate connection 12G of the switch 6, which is in 4 is formed as a further transistor 12, specifically as a further FET 12. Alternatively, the further transistor 12 can also be in the form of a bipolar transistor, optionally also present together with the operational amplifier 35 in an integrated circuit (IC).

A hysteresis behavior in relation to a change in the switching states of the further transistor 12 as a function of the temperature of the thermistor 4 is generated by the Schmitt trigger circuit. In concrete terms, the further transistor 12 is in a blocking switching state when the temperature T of the thermistor 4 is low. As a result, the discharge circuit 100 is operated in the first operating state BZ1 with the transistor 3 switched on. As the temperature of the discharge resistor 2 and/or the transistor 3 increases, the thermistor 4 is heated and the resistance of the thermistor 22 decreases. When the temperature threshold value T _TH is reached or exceeded, there is a sudden change in the output voltage of the operational amplifier 35 from a “low signal” to a “high signal”, as a result of which the further transistor 12 is switched on. As a result, the gate terminal 3G of the transistor 3 is connected to the reference potential GND and the discharge circuit 100 is switched to the second operating state BZ2 when the transistor 3 is off. When the thermistor 4 cools down, on the other hand, the resistance of the NTC thermistor 22 decreases. When a second temperature threshold value T _TH,2 is reached or undershot, there is a sudden change in the output voltage from the “high signal” to the “low signal”, which causes the further Transistor 12 is locked, and the discharge circuit 100 is placed back into the first operating state BZ1. The second temperature threshold value T _TH,2 is smaller than the temperature threshold value T _TH , ie T _TH,2 <T _TH applies. A such hysteresis behavior is usually advantageous with regard to a robust and stable overall operation of the discharge circuit 100 .

Even if the temperature T when the thermistor 4 heats up is greater than or equal to the temperature threshold value T _TH , or the temperature T when the thermistor 4 cools down is greater than or equal to the second temperature threshold value T _TH,2 , repeated alternating changes take place between the second operating state BZ2 that prevails over time when the transistor 3 is off and a first operating state BZ1 that is only briefly assumed when the transistor 3 is switched on. The alternating change results from the following reason: When the switch 6 is switched on, the current applied to the gate connection of the transistor 3 Set DC voltage to the reference potential GND, whereby the operational amplifier 35 loses its electrical supply. It only continues to be supplied for as long as is made possible by a DC voltage present at the buffer capacitor C2 of the voltage stabilization unit 33 . However, since the buffer capacitor C2 discharges, among other things due to energy consumption by the Schmitt trigger circuit 32, the operational amplifier 35 is deactivated when the DC voltage falls below a minimum level and the further transistor 12 as switch 6 is switched back to its blocking switching state. However, this switching state of the switch 6 is only present for a short time if the temperature T of the thermistor 4 does not correspond to permanent operation of the discharge circuit 100 in the first operating state BZ1. Rather, when the switch 6 is briefly off, the gate connection 3G is recharged via the gate charging resistor 9, and thus also the voltage stabilization unit 33, and the switch 6 is switched back to its conducting switching state. In summary, the above-mentioned alternating change between the second operating state BZ2, which is predominantly present in terms of time, and the first operating state BZ1, which is present for a short time, of the discharge circuit 100 also occurs in the fourth specific embodiment of the trigger circuit 5.

Reference List

1

capacity

2

discharge resistance

3

transistor

3G

gate connection

3D

drain connection

3S

source connection

4

Thermistor (PTC, NTC)

5

trigger circuit

6

Switch

7

voltage stabilization unit

9

gate load resistor

10

discharge path

11

thyristor

11G

control port

12

MOSFET

12G

gate connection

13, 14

circuit position (of the trigger circuit)

13a, 13b

connection point (of the trigger circuit)

14a, 14b

connection point (of the trigger circuit)

15a, 15b

connection point (of the trigger circuit)

16a, 16b

connection point (of the trigger circuit)

21

PTC thermistor

22

Thermistor (NTC)

23

optocoupler

28, 29, 30, 31

connection point

32

Schmitt trigger circuit

33

voltage stabilization unit

35

operational amplifier

ZD1 - ZD6

zener diode

R1-R11

Resistance

D1, D2

diode

C1, C2

buffer capacitor

TTH, TTH,2

temperature threshold

100

discharge circuit

101

Entry

101a, 101b

input port

S1-S8

process step

Claims (18)

Discharge circuit (100) for discharging a capacitance (1), comprising - an input (101) with a first input connection (101a) and a second input connection (101b) for connection to a respective connection of the capacitance (1), - a transistor (3) whose drain terminal (3D) is connected to the first input terminal (101a) via a discharge resistor (2) and whose source terminal (3S) is connected to the second input terminal (101b), - a gate charging resistor (9) connecting the first input terminal (101a) to a gate terminal (3G) of the transistor (3), - a thermistor (4) thermally coupled to the discharge resistor (2) and/or the transistor (3), and - a switch (6) with a conducting and a blocking switching state, which connects the gate terminal (3G) to the second input terminal (101b), and is designed to change its switching state in response to a trigger signal, and - a trigger circuit (5 ) for controlling the switch (6), which is designed to generate the trigger signal for the switch (6) depending on a temperature of the thermistor (4), characterized in that the discharge circuit (100) is set up from one at the input terminals (101a, 101b) applied voltage to be supplied, and that - the switch (6) and/or a combination of the trigger circuit (5) and the switch (6) is set up to change the switching state of the switch (6) in response to the To change the trigger signal abruptly from the blocking to the conducting switching state in order to generate a transient change from a first operating state BZ1 of the discharge circuit (100) when the transistor (3) is turned on to a second operating state BZ2 of the discharge circuit (100) when the transistor (3) is blocking . Discharge circuit (100) after claim 1 , characterized in that the switch (6) is bistable and comprises, for example, a thyristor (11). Discharge circuit (100) after claim 1 , characterized in that the switch (6) comprises a transistor (12), in particular a MOSFET transistor or an IGBT transistor, and the trigger circuit (5) is designed as a flip-flop. Discharge circuit (100) according to one of Claims 1 until 3 , characterized in that the thermistor (4) comprises a thermistor (22). Discharge circuit (100) according to one of Claims 1 until 3 , characterized in that the thermistor (4) comprises a PTC thermistor (21). Discharge circuit (100) according to one of Claims 1 until 5 , characterized in that the thermistor (4), optionally also the trigger circuit (5), is arranged in a discharge path (10) which is formed by the series connection of discharge resistor (2) and transistor (3). Discharge circuit (100) according to one of the preceding claims, characterized in that the discharge circuit (100) additionally has a voltage stabilization unit (7) which contains a zener diode (ZD1) or a parallel circuit made up of a zener diode (ZD1) and a buffer capacitor (C1), wherein the cathode of the zener diode (ZD1) is connected to the gate terminal (3G) of the transistor (3) and the anode of the zener diode (ZD1) is connected to the second input terminal (101b). Discharge circuit (100) according to one of Claims 5 until 7 , characterized in that the switch (6) is designed as a thyristor (11) and the trigger circuit (5) has a parallel circuit made up of the PTC thermistor (21) and a series circuit made up of a second Zener diode (ZD2) and a second resistor (R2), a connection point (28) between the second zener diode (ZD2) and the second resistor (R2) being connected to a control connection (11G) of the switch (6) designed as a thyristor (11). Discharge circuit (100) according to one of Claims 1 until 7 , characterized in that the trigger circuit (5) has a parallel connection of a third zener diode (ZD3) and a series connection of the thermistor (22) and a third resistor (R3), with a connection point (29) between the thermistor (22) and the third resistor (R3) is connected to a control terminal (11G) of the switch (6) designed as a thyristor (11). Discharge circuit (100) according to one of Claims 1 until 7 , characterized in that the trigger circuit (5) comprises an optocoupler (23). Discharge circuit (100) according to one of Claims 1 until 7 , characterized in that the trigger circuit (5) is designed as a flip-flop and in particular comprises a Schmitt trigger circuit (32). Discharge circuit (100) according to one of the preceding claims, characterized in that the discharge resistor (2) comprises a multiplicity of resistors which are connected in series and/or in parallel with one another. Method for discharging a capacitance (1) via a discharging circuit (100) according to one of the preceding claims, which has a first operating state BZ1 with an open switch (6) and turned on transistor (3), and a second operating state BZ2 with closed switch (6) and blocked transistor (3), with the steps: - Operating the discharge circuit (100) depending on a temperature of the thermistor (4) in one of the first BZ1 and the second operating state BZ2, - wherein the discharge circuit (100) is operated in a stationary manner in the first operating state BZ1 when a temperature of the thermistor (4) is below a temperature threshold value T _TH , and - wherein the discharge circuit 100 has a repeated change between the first operating state BZ1 and the second operating state BZ2 is operated when the temperature of the thermistor (4) is greater than or equal to the temperature threshold value T _TH , so that the transistor (3) in a repeated manner between the controlled and the locked state changes procedure after Claim 13 , wherein the stationary operation of the discharge circuit (100) in the first operating state BZ1 only takes place when the temperature of the thermistor (4) has fallen below a second temperature threshold value T _TH,2 which is lower than the temperature threshold value T _TH , and wherein operation of the discharge circuit (100) at a temperature of the thermistor (4) between the temperature threshold value T _TH and the second temperature threshold value T _TH,2 depends on whether the thermistor (4) is cooling down or heating up. procedure after Claim 13 or 14 , wherein the operation of the discharge circuit (100) as a function of a temperature of the thermistor (4) lasts until a DC voltage dropping across the capacitor (1) falls below a voltage threshold value UTH. Electrical device, characterized in that the device has a capacity (1) and a capacity (1) acting on the discharge circuit (100) according to one of Claims 1 until 11 includes, and to carry out the method Claim 13 or Claim 14 set up, and - wherein the device is set up by means of the discharge circuit (100) to discharge the device-internal capacitance (1) even in a state in which the device is separated from its energy supply. Electrical device after Claim 16 , characterized in that the device is designed as a photovoltaic (PV) inverter, in which the capacitance (1) is formed via one or more input capacitances of the PV inverter that are decoupled by diodes. Electrical device after Claim 16 or 17 , characterized in that the device has a control circuit which acts on the gate terminal (3G) of the transistor (3) and can be controlled by a controller of the device and which is set up to discharge the capacitance (1) as a function of a control signal from the device-internal controller suppress or enable.

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